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The ast module helps Python applications to process trees of the Python
abstract syntax grammar. The abstract syntax itself might change with each
Python release; this module helps to find out programmatically what the current
grammar looks like.

An abstract syntax tree can be generated by passing ast.PyCF_ONLY_AST as
a flag to the compile() built-in function, or using the parse()
helper provided in this module. The result will be a tree of objects whose
classes all inherit from ast.AST. An abstract syntax tree can be
compiled into a Python code object using the built-in compile() function.

This is the base of all AST node classes. The actual node classes are
derived from the Parser/Python.asdl file, which is reproduced
below. They are defined in the _ast C
module and re-exported in ast.

There is one class defined for each left-hand side symbol in the abstract
grammar (for example, ast.stmt or ast.expr). In addition,
there is one class defined for each constructor on the right-hand side; these
classes inherit from the classes for the left-hand side trees. For example,
ast.BinOp inherits from ast.expr. For production rules
with alternatives (aka “sums”), the left-hand side class is abstract: only
instances of specific constructor nodes are ever created.

Each concrete class has an attribute _fields which gives the names
of all child nodes.

Each instance of a concrete class has one attribute for each child node,
of the type as defined in the grammar. For example, ast.BinOp
instances have an attribute left of type ast.expr.

If these attributes are marked as optional in the grammar (using a
question mark), the value might be None. If the attributes can have
zero-or-more values (marked with an asterisk), the values are represented
as Python lists. All possible attributes must be present and have valid
values when compiling an AST with compile().

Instances of ast.expr and ast.stmt subclasses have
lineno and col_offset attributes. The lineno is
the line number of source text (1-indexed so the first line is line 1) and
the col_offset is the UTF-8 byte offset of the first token that
generated the node. The UTF-8 offset is recorded because the parser uses
UTF-8 internally.

The constructor of a class ast.T parses its arguments as follows:

If there are positional arguments, there must be as many as there are items
in T._fields; they will be assigned as attributes of these names.

If there are keyword arguments, they will set the attributes of the same
names to the given values.

For example, to create and populate an ast.UnaryOp node, you could
use

Safely evaluate an expression node or a string containing a Python literal or
container display. The string or node provided may only consist of the
following Python literal structures: strings, bytes, numbers, tuples, lists,
dicts, sets, booleans, and None.

This can be used for safely evaluating strings containing Python values from
untrusted sources without the need to parse the values oneself. It is not
capable of evaluating arbitrarily complex expressions, for example involving
operators or indexing.

Return the docstring of the given node (which must be a
FunctionDef, ClassDef or Module node), or None
if it has no docstring. If clean is true, clean up the docstring’s
indentation with inspect.cleandoc().

When you compile a node tree with compile(), the compiler expects
lineno and col_offset attributes for every node that supports
them. This is rather tedious to fill in for generated nodes, so this helper
adds these attributes recursively where not already set, by setting them to
the values of the parent node. It works recursively starting at node.

Recursively yield all descendant nodes in the tree starting at node
(including node itself), in no specified order. This is useful if you only
want to modify nodes in place and don’t care about the context.

A NodeVisitor subclass that walks the abstract syntax tree and
allows modification of nodes.

The NodeTransformer will walk the AST and use the return value of
the visitor methods to replace or remove the old node. If the return value
of the visitor method is None, the node will be removed from its
location, otherwise it is replaced with the return value. The return value
may be the original node in which case no replacement takes place.

Here is an example transformer that rewrites all occurrences of name lookups
(foo) to data['foo']:

Return a formatted dump of the tree in node. This is mainly useful for
debugging purposes. The returned string will show the names and the values
for fields. This makes the code impossible to evaluate, so if evaluation is
wanted annotate_fields must be set to False. Attributes such as line
numbers and column offsets are not dumped by default. If this is wanted,
include_attributes can be set to True.

See also

Green Tree Snakes, an external documentation resource, has good
details on working with Python ASTs.